Intrigued by dramatic photographs showing lightning within the plume created by the eruption of Chile’s Mount Chaiten last year (above), academics from the University of Illinois at Urbana-Champaign sought out similar records from other eruptions in the hope of uncovering the mechanism for the generation of the lightning ‘sheaths’.

Using a combination of historical records and satellite images, the team was able to show that the plumes of past eruptions have indeed exhibited similar behaviour and that it’s triggered by the rotation of the entire plume.

‘Classical models of volcanic plumes consist of a vertical column topped by a horizontal umbrella,’ said Pinaki Chakraborty, a postdoctoral research geologist and
the lead author of the paper. ‘But we have shown that these models are missing a subtle but crucial feature: a “volcanic mesocyclone” that sets both column and
umbrella spinning about their vertical axis.’ 

This rotation of the plume is thought to trigger lightning, dust devils and waterspouts in much the same way as tornados. ‘What happens in tornadic thunderstorms is analogous to what happens in strong volcanic plumes,’ said Chakraborty. ‘The mesocyclone spawns waterspouts or dust devils, and groups the electric charges in the plume to form a sheath of lightning, as was so prominently displayed in the eruption of Mount Chaiten.’

The team hopes that research into volcanic mesocyclones will help future efforts in forecasting and alleviating the impact of volcanic eruptions.

June 2009